It has been well established in a number of biological systems that cytoplasmic regulatory factors play a cardinal role in regulating nuclear gene activity and nuclear reprogramming. Initially it was shown that transfer of nuclei from fully differentiated cells of frog into enucleated frog oocytes resulted in the development of tadpoles to mature frogs [Briggs, R. and T. J. King, Transplantation of Living Nuclei From Blastula Cells into Enucleated Frogs' Eggs. Proc Natl Acad Sci USA, 1952. 38(5): p. 455-63; Gurdon, J. B., Adult frogs derived from the nuclei of single somatic cells. Dev Biol, 1962. 4: p. 256-73; Gurdon, J. B., Nuclear transplantation in Xenopus. Methods Mol Biol, 2006. 325: p. 1-9; Gurdon, J. B. and J. A. Byrne, The first half-century of nuclear transplantation. Biosci Rep, 2004. 24(4-5): p. 545-57.].
Successful generation of embryonic stem (ES) cells of different species by Somatic Cell Nuclear Transfer (SCNT) into enucleated eggs has firmly established that different cell types arise from epigenetic changes and not from any permanent change in the DNA sequence [Byrne, J. A., et al., Producing primate embryonic stem cells by somatic cell nuclear transfer. Nature, 2007. 450(7169): p. 497-502; Hochedlinger, K. and R. Jaenisch, Monoclonal mice generated by nuclear transfer from mature B and T donor cells. Nature, 2002. 415(6875): p. 1035-8; Wilmut, I., et al., Viable offspring derived from fetal and adult mammalian cells. Nature, 1997. 385(6619): p. 810-3.]. Generation of induced Pluripotent Stem (iPS) cells has also shown that epigenetic changes can be brought out by forced expression of a few genes introduced exogenously into adult cells [Aoi, T., et al., Generation of pluripotent stem cells from adult mouse liver and stomach cells. Science, 2008. 321(5889): p. 699-702; Lowry, W. E., et al., Generation of human induced pluripotent stem cells from dermal fibroblasts. Proc Natl Acad Sci USA, 2008. 105(8): p. 2883-8; Meissner, A., M. Wernig, and R. Jaenisch, Direct reprogramming of genetically unmodified fibroblasts into pluripotent stem cells. Nat Biotechnol, 2007. 25(10): p. 1177-81; Nakagawa, M., et al., Generation of induced pluripotent stem cells without Myc from mouse and human fibroblasts. Nat Biotechnol, 2008. 26(1): p. 101-6; Takahashi, K., et al., Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell, 2007. 131(5): p. 861-72; Takahashi, K. and S. Yamanaka, Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell, 2006. 126(4): p. 663-76; Yamanaka, S., Induction of pluripotent stem cells from mouse fibroblasts by four transcription factors. Cell Prolif, 2008. 41 Suppl 1: p. 51-6; Yu, J., et al., Induced pluripotent stem cell lines derived from human somatic cells. Science, 2007. 318(5858): p. 1917-20.].
Tissue culture cell lines that express functions characteristic of the tissues from which they were established have been used to study the regulation of differentiated functions in vitro. Somatic cell hybrids between different cell types have shown that differentiated functions can either be activated or extinguished in such cell hybrids depending upon the cell types used in these hybrids [Cowan, C. A., et al., Nuclear reprogramming of somatic cells after fusion with human embryonic stem cells. Science, 2005. 309(5739): p. 1369-73; Davis, F. M. and E. A. Adelberg, Use of somatic cell hybrids for analysis of the differentiated state. Bacteriol Rev, 1973. 37(2): p. 197-214; Wright, W. E. and J. Aronoff, The suppression of myogenic functions in heterokaryons formed by fusing chick myocytes to diploid rat fibroblasts. Cell Differ, 1983. 12(5): p. 299-306; Tada, M., et al., Nuclear reprogramming of somatic cells by in vitro hybridization with ES cells. Curr Biol, 2001. 11(19): p. 1553-8; Terranova, R., et al., Acquisition and extinction of gene expression programs are separable events in heterokaryon reprogramming. J Cell Sci, 2006. 119(Pt 10): p. 2065-72.]. Such activation and extinction of differentiated functions were also seen in heterokaryons immediately after cell fusion before formation of synkaryons [Terranova, R., et al., Acquisition and extinction of gene expression programs are separable events in heterokaryon reprogramming. J Cell Sci, 2006. 119(Pt 10): p. 2065-72; Baron, M. H. and T. Maniatis, Rapid reprogramming of globin gene expression in transient heterokaryons. Cell, 1986. 46(4): p. 591-602; Blau, H. M., C. P. Chiu, and C. Webster, Cytoplasmic activation of human nuclear genes in stable heterokaryons. Cell, 1983. 32(4): p. 1171-80; Pereira, C. F., et al., Heterokaryon-based reprogramming of human B lymphocytes for pluripotency requires oct4 but not sox2. PLoS Genet, 2008. 4(9): p. e1000170; Wright, W. E., The isolation of heterokaryons and hybrids by a selective system using irreversible biochemical inhibitors. Exp Cell Res, 1978. 112(2): p. 395-407; Wright, W. E., Induction of myosin light chain synthesis in heterokaryons between normal diploid cells. In Vitro, 1982. 18(10): p. 851-8.], suggesting the role of cytoplasmic regulatory factors responsible for nuclear gene activity.
Other studies investigated the role of cytoplasmic regulatory factors in regulating nuclear gene activity and including development of tools to enucleate cells and form hybrids between such enucleated cells, called cytoplasts, and another whole cell. The resulting cytoplast-whole cell hybrids were called cybrids [Bunn, C. L., D. C. Wallace, and J. M. Eisenstadt, Cytoplasmic inheritance of chloramphenicol resistance in mouse tissue culture cells. Proc Natl Acad Sci USA, 1974. 71(5): p. 1681-5.]. It has been shown that cytoplasmic regulatory factors bring about permanent activation as well as extinction of differentiated functions in somatic cell cybrids, and such factors are cytoplasmically inheritable [Gopalakrishnan, T. V. and W. F. Anderson, Epigenetic activation of phenylalanine hydroxylase in mouse erythroleukemia cells by the cytoplast of rat hepatoma cells. Proc Natl Acad Sci USA, 1979. 76(8): p. 3932-6; Gopalakrishnan, T. V., E. B. Thompson, and W. F. Anderson, Extinction of hemoglobin inducibility in Friend erythroleukemia cells by fusion with cytoplasm of enucleated mouse neuroblastoma or fibroblast cells. Proc Natl Acad Sci USA, 1977. 74(4): p. 1642-6.].
Since cell enucleation was an inefficient process and could not be applied uniformly to different cell types, chemical means were used to enucleate cells functionally rather than physically to totally exclude nuclear gene contribution from cytoplasmic donor cells during this activation process [Gopalakrishnan, T. V. and J. W. Littlefield, RNA from rat hepatoma cells can activate phenylalanine hydroxylase gene of mouse erythroleukemia cells. Somatic Cell Genet, 1983. 9(1): p. 121-31.]. It was shown in these studies that treatment of cells with a high concentration of Mitomycin C completely prevented contribution of single selectable gene marker necessary for the survival of hybrids following fusion with a partner cell deficient in the same selectable marker gene. These studies led to establishing conditions to investigate the role of non genetic components of a cell in regulating nuclear gene activity [Gopalakrishnan, T. V. and J. W. Littlefield, RNA from rat hepatoma cells can activate phenylalanine hydroxylase gene of mouse erythroleukemia cells. Somatic Cell Genet, 1983. 9(1): p. 121-31.]. These were termed as “pseudocybrids.” These studies showed that cytoplasmic regulatory factors for a liver specific marker, phenylalanine hydroxylase, exist in liver cell lines that express phenylalanine hydroxylase constitutively, which when introduced into a suitable non-hepatic recipient cell line by formation of cybrids or pseudocybrids, can bring about permanent activation of phenylalanine hydroxylase gene from the genome of the non-hepatic cell [Gopalakrishnan, T. V. and J. W. Littlefield, RNA from rat hepatoma cells can activate phenylalanine hydroxylase gene of mouse erythroleukemia cells. Somatic Cell Genet, 1983. 9(1): p. 121-311. The resulting cybrids could be used as cytoplasmic donor cells to epigenetically activate phenylalanine hydroxylase gene in different recipient cells to generate second generation cybrids. These studies also showed that the cytoplasmic activation was brought about by RNA or perhaps a protein coded by it, and that the cytoplasmic regulatory factor was perpetuated continuously in culture.
Epigenetic mechanisms are responsible for development of different cells types from a fertilized egg. Somatic cell nuclear transfer (SCNT) experiments have established that differentiated cell types arise from reversible epigenetic control mechanisms rather than from any permanent alteration in the genetic component. Embryonic stem (ES) cells of various mammalian species have been developed by SCNT into unfertilized eggs. These cells arise from epigenetic changes caused by egg cytoplasm to donor nucleus genome. Recently, ES like cells, referred to as induced pluripotent stem (iPS) cells, have been generated by direct reprogramming of adult cells following introduction of a few specific genes into them. This has opened up opportunities to generate patient-specific iPS cells that can be used to study the disease process, for drug discovery and drug toxicology studies, and for regenerative medicine and cell therapy applications. However, generation of specific cell types either from ES cells or iPS cells as a pure population in sufficient quantity to meet the demand for various applications has thus far remained inefficient.